Arc discharge lamps operate by jumping an electric arc between two displaced electrodes. The electric arc excites the atoms of a gas along the arc path to high energy states resulting in light and heat. Arcing ideally occurs directly between the electrode tips, which is generally accomplished by making the gap between the electrode tips the smallest distance between the electrodes. In single ended arc tubes, the electrodes are substantially side by side and parallel. The tip to tip distance may then be reduced by angling the electrodes towards each other, as seen in U.S. Pat. Nos. 4,557,700 to Rothwell et al or in 4,612,000 to English et al. Another method is to bend the tips over as seen in U.S. Pat. Nos. 4,415,829 to Rothwell, Jr et al. or 4,308,483. A feature may be created on the electrode end, such as a ball or loop (see U.S. Pat. No. 4,320,322 Rothwell Jr. et al.) to narrow the distance interelectrode distance. Theoretically, arcing is then most likely to occur at the tips. The arc path is then at the electrode tips, well away from the envelope wall.
Despite the methods of angling and bending the electrodes, straight electrodes are still used for ease of manufacture, cost, alignment and other more practical reasons, particularly for mall volume lamps, as for example seen in U.S. Pat. No. 4,321,504 to Keefe et al.
A problem in single ended arc lamps is arcing misplaced from the electrode tips. In particular, arcing can move from the electrode tips, to the electrode roots, where the electrodes emerge from the envelope material. Arcing at the electrode roots occurs particularly during a hot restart of the lamp. A surface effect makes arcing along the envelope surface easier than in the open volume between the electrode tips. The surface effect is thought to result from a lower molecular activity in the gas near the envelope surface which lowers electrical resistance, and from the molecular alignment of the envelope surface material which allows electrons to be given up more easily which also lowers resistance. Due to the small enclosed volume in smaller arc capsules, the electrode placement and separation is limited. As a result, the surface effect cannot always be fully offset by separating or angling the electrodes. Small lamps in particular are then more subject to surface arcing at the seal area instead of at the electrode tips.
Discharge between the electrode roots is undesirable for at least two reasons. First, the arc is moved from the expected point of formation. The resulting light is then offset from the light path planned for focusing and projection. Second, the heat loading of the envelope adjacent the arc discharge is increased, and may rise above the material limit for the envelope. The lamp can then fail in the electrode root area due to the excess heating by the surface arcing.